This invention concerns a method of depositing a film of tungsten which is used as the gate metal in semiconductor devices.
In the prior art, films of tungsten (W), a gate metal in semiconductor devices, were generally formed on a film of substrate gate oxide of SiO.sub.2 by means of sputtering.
This method however suffered from the following disadvantages:
(i) Step coverage over the SiO.sub.2 step is poor, PA1 (ii) Damage to the gate oxide due to sputtering occurs easily, PA1 (iii) There is a risk that impurities in the target material will contaminate the W film, and PA1 (iv) Problems of properties such as high resistance occur easily.
CVD (Chemical Vapor Deposition) of the W film was therefore proposed as one way of eliminating these problems.
In the deposition of these W films by CVD, tungsten hexafluoride (WF.sub.6) and hydrogen (H.sub.2) are used as reaction gases. By the usual thermal CVD method, however, W films are not deposited on a silicon oxide film.
It was therefore proposed to deposit the W film by laser CVD instead of thermal CVD, as disclosed in the literature (A. Shintani, J. Appl. Physics, Vol. 61, No. 6, March 1987, p. 2365-2366). (The laser CVD will be described in detail later).
By means of the laser CVD method, it is possible to deposit a W film on the surface of a silicon oxide film. The adhesion between the film and the W film is however poor, and under certain conditions, the W film may peel off at the interface with the silicon oxide film or cracks may occur in the W film as it is being formed.
In the method of the above reference, it is proposed that a silicon substrate covered by a silicon oxide film be heated to 350.degree. C. or more, and the H.sub.2 /WF.sub.6 flow ratio be no less than 2, in order to form a stable W film on the silicon oxide.
It has been however found that if the W film was deposited by the above laser CVD method under the above conditions, the W film tended to peel off at the interface with the SiO.sub.2 film as film thickness increased when the tensile stress in the W film was large, and it was therefore difficult to deposit a stable film on the silicon oxide on the substrate.